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313
METALLURGY OF IBON AND STEEL.
of cementite. At about 1050° C., however, cementite as such disappears even in high carbon steels and the carbon is considered as being in solution in gamma iron. This is the point above which it is necessary to heat in order to obtain austenite, from which it is argued that austenite is carbon dissolved in gamma iron.
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1400° 1300° 1200C 1100° 1000° 900° 800° 700° 600°	\	\:		CL									/	/
		p\		\	\			jiqiii(				^	/	
			\	L	quid	•v^jj	"s>^			U _/	/	Liquid + G(raphlt		i
				Sj-Ma	rtensi	e								
		7		\		*		^\	^	/	V			G
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		Mart<	nsite	sL	>		M(	trtens:	te^.	^x..	"^	*"    ^	~)i	F'
							G	raphi	te					
	G		/	/		Marte	nsite 4	.Ceine	ntite					"   H a m
	h		fm											.1
	M&^ P	X/					rt,							* rl
	Fen	ite-t			Pearl	ite+C	iment	te						% $
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FlG. XV-H.—GrEAPHICAL EEPEESENTATION OF THE PHASE DOCTRINE*
Martensite is considered as a solution of Fe3C in allotropic iron, being a saturated solution in steel containing about .89 per cent, carbon.
Prof. Arnold has disputed the allotropic theory in several articles and has evolved an hypothesis of his own which he calls the "sub-carbide theory/' on the supposition that hardening is due to the retention of a hard sub-carbide of iron Fe24Ct
These theories will be found thoroughly considered in the volumes of the Iron and Steel Institute of the past few years. Enough is given here to show the variety of ideas, all of which have their strong and their weak points,